Mitigation and elimination of tin whiskers

ABSTRACT

A method of mitigating tin whisker formation on electronic assemblies includes exposing tin metal in the electronic assembly to a mitigating agent that interacts with the tin metal to produce a product that is resistant to forming tin whiskers.

BACKGROUND

The present invention relates to the manufacture of electronicassemblies, and more specifically, to the prevention and/or removal oftin whiskers on the electronic assemblies.

According to one embodiment of the present invention, the formation oftin whiskers may be addressed via the conversion of tin metal in theelectronic assembly to a material that is resistant to forming whiskers.This conversion may occur via treatment of the electronic assembly witha mitigating agent that interacts with the tin metal present, or viaincorporation of an electronic assembly with a mitigating agent thatinteracts with tin metal.

BRIEF SUMMARY

In one embodiment, the present invention includes a method of mitigatingtin whisker formation on electronic assemblies that includes providingan electronic assembly that includes an electrical connection having atleast a surface that is substantially pure tin metal, exposing the tinmetal to a mitigating agent. That mitigating agent is selected tointeract with the tin metal, under appropriate conditions that promotethe desired interaction, thereby to produce a product that is resistantto forming whiskers,

In another embodiment of the present invention, a whisker-resistantelectronic assembly may be prepared by providing an electronic assemblythat has an electrical connection that includes substantially pure tinmetal, exposing the tin metal to a mitigating agent selected to interactwith the tin metal to produce a product that is resistant to formingwhiskers, and curing the electronic assembly under conditions selectedto promote the desired interaction.

In yet another embodiment of the present invention, an electronicassembly may include a circuit board substrate, a conducting trace thatincludes copper metal, and an electrical component coupled to theconducting trace via tin metal, where the conducting trace and the tinmetal are coupled via a mitigating metal including at least one ofgallium, indium, gold, copper, aluminum, silver, magnesium, nickel,zinc, and lead, provided the mitigating metal is present in sufficientquantity that upon heating, the mitigating metal substantially convertsthe tin solder to an alloy that resists whisker formation.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 includes a flowchart that illustrates an exemplary embodiment ofa method of mitigating tin whisker formation according to the presentinvention.

FIG. 2 includes a plan view of a schematic representing an exemplaryembodiment of an electronic assembly according to the present invention.

FIG. 3 depicts a detail of the electronic assembly of FIG. 2, in aperspective view.

FIG. 4 includes a cross-sectional view of a detail of an electronicconnection in the electronic assembly of FIG. 3, after preparation butprior to curing of that assembly.

FIG. 5 is a detail of the electronic assembly of FIG. 4 during a curingprocess, showing partial conversion of tin metal to whisker-resistantproducts, according to an exemplary embodiment of the invention.

FIG. 6 is a detail of the electronic assembly of FIG. 4 after completinga curing process, and showing complete conversion of tin metal towhisker-resistant products, according to another exemplary embodiment ofthe invention.

FIG. 7 includes a cross-sectional view of a detail of an electronicconnection in an alternative electronic assembly, after preparation butprior to curing of that assembly, according to an exemplary embodimentof the invention.

FIG. 8 is a detail of the electronic assembly of FIG. 7 after completinga curing process, and showing complete conversion of tin metal towhisker-resistant products, according to another exemplary embodiment ofthe invention.

DETAILED DESCRIPTION

FIG. 1 depicts a flowchart 10 setting out a method of mitigating tinwhisker formation in an electronic assembly. The method includesproviding an electronic assembly that has at least one electricalconnection that includes at least one surface that is substantially puretin metal, as shown at 12. Specifically, the method includes exposingthe tin metal to at least one mitigating agent that is selected tointeract with the tin metal to produce a product that is resistant toforming whiskers, where the electronic assembly is exposed to themitigating agent under appropriate conditions to promote the desiredinteraction, as shown at 14.

A schematic depicting an exemplary electronic assembly 16 is shown inFIG. 2. As used herein, an electronic assembly 16 is a collection of oneor more electronic components 18 that are electrically connected to aconductive pathway, track, or signal trace, 20. The electronicassemblies discussed in this disclosure typically include at least oneelectrical connection 22 that includes at least one surface that issubstantially pure tin metal. Typically, the electronic assemblyincludes a nonconductive substrate 24 that supports the one or moreelectronic components, as well as the electrical connection. In oneembodiment, the electronic assembly includes a substrate that is aprinted circuit board, or PCB, that mechanically supports andelectrically connects the electronic components using conductivepathways, tracks or signal traces. In another particular embodiment, theelectronic assembly corresponds to a printed circuit board and aspecified arrangement of one or more electronic components prior to asolder reflow process. In another particular embodiment, the electronicassembly corresponds to a printed circuit board and a specifiedarrangement of one or more electronic components after a solder reflowprocess.

The printed circuit board may be prepared by bonding a thin layer of aconductive metal onto the substrate, and then removing unwanted metal bymasking and etching. Alternatively, the printed circuit board may beprepared by adding conductive traces to the nonconductive substrate by aprocess of electroplating. The conducting traces typically includecopper. A printed circuit board that supports and interconnects aplurality of electronic components may be referred to as a printedcircuit board assembly, or PCBA.

As used herein, an electronic component 18 is an electronic elementhaving two or more electrical connections. Electronic components aretypically connected, such as by attachment to a printed circuit board,so as to create an electronic circuit with a particular desiredfunction. Examples of electronic components include, for example,resistors, capacitors, transistors, diodes, fuses, inductors,transducers, and the like. Electronic components may also refer to morecomplex devices such as integrated circuits (or chips).

A region of the electronic assembly 16 of FIG. 2 is shown in greaterdetail in FIG. 3. In FIG. 3, electronic components 18 include at leastone electrical connection 22 between the electronic component 18 andconductive trace 20. Although the electrical connection typically isbetween an individual component and a conductive trace that is presenton a printed circuit board, the specific nature of the electricalconnection may vary widely. In general, any type of electricalconnection that results in a suitable electronic assembly for thepurposes of the instant method is an appropriate electrical connection.

As shown in FIG. 3, and in greater detail in the cross-sectional view ofFIG. 4, electrical connection 22 may include one or more leads 26 of agiven component through an aperture or hole 28 formed in substrate 24,where the sides of the hole may include copper metal 30 (through-holeassembly). Alternatively, the leads of a given electronic component maybe placed on lands or pads on a PCB. In either case, the component leadsare subsequently fixed to the circuit board with a soft metal solder. Inyet another alternative aspect, the electronic component may beconnected to a printed circuit board via one or more balls of solder,for example via a ball grid array, where the component is a chip, a chiparray, a flip chip, and the like.

The solder used to form the electrical connections in the electronicassembly typically includes both tin and lead metals. Tin/lead solders,however, are typically not susceptible to whisker formation. Only inrelatively few cases is pure tin solder is used. In addition, electronicassemblies may include plain copper metal, such as typically makes upconductive traces. As the copper is prone to oxidation, the copper mustbe coated by a component finish, which is often a tin metal finish. Inaddition, a variety of electronic components may incorporate electricalconnections that include pure tin metal, for example surface finishes ofpure tin metal, and are therefore prone to tin whisker formation.

An exemplary connection 22 between an electronic component and asubstrate 24 prior to curing of the electronic assembly is depictedschematically in FIG. 4, and includes metal solder 32 forming aconnection between component lead 26 of a representative electroniccomponent and a conductive trace 20 of copper 30 that extends around andwithin aperture 28 in substrate 24. Component lead 26 includes arepresentative component finish with a finishing layer 33 of pure tinmetal, as does conductive trace 20 of copper metal 30 leading leftwardfrom connection 22. In the absence of the appropriate conditions topromote the desired interaction with the selected mitigating agent, suchas via a curing process, the pure tin metal of connection 22, present onboth the component lead and the conductive traces, would remainsusceptible to whisker formation and potential failure.

As discussed above with respect to FIG. 1, the method depicted inflowchart 10 includes exposing the tin metal present in an electricalconnection of the electronic assembly to at least one mitigating agentselected to interact with the tin metal to produce a product that isresistant to forming whiskers (at 34 of flowchart 10). In one embodimentof the method, the mitigating agent is a reagent, such that exposing thetin metal of the electronic assembly to the reagent produces a reactionproduct that is resistant to forming whiskers. Typically, theinteraction between the reagent and the tin metal is a chemicalreaction, and occurs under conditions selected to promote the desiredchemical reaction. Such conditions may include any combination of apreferred temperature or temperature range, a preferred appliedpressure, applying the reagent as a solution, applying the reagent as avapor, and/or applying the reagent in the presence of one or moreadditional reagents.

In one embodiment of the method, the reagent may be applied as a vaporor gas. Such an application may take place in a conventional vaporchamber, and may employ a procedure that includes placing the electronicassembly into the vapor chamber, purging the ambient atmosphere from thevapor chamber, exposing the electronic assembly to one or more desiredreagents as gas or vapor for a time sufficient to convert the tin metalpresent to a product that is resistant to whisker formation, followed byventing or purging the reagent gas or vapor. This treatment of theelectronic assembly is optionally combined with heating and/or coolingthe electronic assembly.

In one embodiment of the method the mitigating agent is oxygen, and theinteraction between the oxygen and the tin metal is an oxidationreaction. Typically, oxidation of tin metal according to the presentmethod results in the formation of tin oxides, more specifically theformation of at least one of SnO and SnO₂, which are resistant towhisker formation. The interaction of oxygen and the tin metal presenton the electronic assembly may be facilitated by heating the electronicassembly above room temperature in the presence of oxygen. For example,the electronic assembly may be heated to between about 25° C. and about90° C. to facilitate oxidation of tin metal. In another embodiment ofthe method the mitigating agent is chlorine gas, or other chlorinatingagent, and the tin metal present in the electronic assembly is convertedto tin (I) chloride and/or tin (II) chloride.

Where the tin metal of the electronic assembly is exposed to a reagent,the exposure conditions may be selected to convert a region of the tinmetal surface to a product that is resistant to forming whiskers. Thatis, the exposed surfaces of pure tin metal present in the electronicassembly is passivated with respect to whisker formation by completelyencapsulating the exposed tin metal with a layer, or skin, of thereaction product. An example of this type of surface mitigation isdepicted in FIG. 5, where exposure to an appropriate mitigating reagentunder appropriate conditions has converted a surface layer of tin metal33 present on component lead 26 into a product 36 that is resistant towhisker formation. A similar conversion is shown for the silver finishpresent on conductive trace 20. This skin-forming process passivates theremainder of the pure tin metal present on component lead 26 withrespect to whisker formation. At this point, mitigation may be halted orcontinued until substantially the entire thickness of the tin metal isconverted to a whisker-resistant product (FIG. 6).

Where further conversion is desired, the exposure conditions and reagentused may be selected so that additional exposure to the mitigatingreagent converts the entire thickness of tin metal to a whiskerresistant product. As shown in FIG. 6, after the desired reaction and/orcuring process is substantively complete, tin metal 33 present oncomponent lead 26 of the electronic assembly has been converted towhisker-resistant product 36.

Alternatively, or in addition, as shown at 38 in FIG. 1, the disclosedmethod optionally includes exposing the tin metal present in anelectrical connection of the electronic assembly to at least onemitigating agent that is a metal, with the interaction of the mitigatingagent and the tin metal forming an alloy of tin that is resistant toforming whiskers.

In one embodiment of the invention, depicted in FIG. 4, the mitigatingagent is a conducting layer of copper metal, such as copper metal 30forming conductive trace 20 on substrate 24. In this embodiment, theformation of the alloy of tin includes the formation of one or more ofCu₆Sn₅ and CuSn₃, which alloys are not prone to forming whiskers. Asdepicted in FIG. 5, the interaction of tin metal 33 with copper metal 30results in the partial conversion of the tin metal to whisker-resistantcopper alloy 40 during the curing process.

In one particular embodiment of the invention, the electronic assemblyinteracts with at least two mitigating agents, as depicted in FIGS. 4-6.One of those agents is surface-interactive, resulting in a reaction thatprogresses inwardly into and throughout the tin phase until all the tinpresent is converted, while the other agent reacts to cause a conversionat the tin/metal interface from a tin material to a whisker-resistantalloy, with alloy formation then extending throughout the tin phase.Specifically, a first mitigating agent, such as an applied reagent,converts tin metal at an exposed tin metal surface, and proceedsthroughout the contiguous tin phase. Internal to that surface, and at ametal-tin interface, there is an interaction of tin metal with themitigating agent that simultaneously or sequentially converts the tinmetal to a whisker-resistant alloy. Each conversion may proceed untilall free tin has been converted to a whisker-resistant product, as shownin FIG. 6.

In an alternative embodiment, as depicted in FIG. 7 for an electronicassembly before curing, a mitigating agent formed as a metal film 42 isapplied to copper metal 30 that is typically formed as conductive traces20 of an electronic assembly, as well as component lead 26. Duringsubsequent application of the component finish, pure tin metal 33 formsa film on mitigating agent film 42 where it has been applied. As it isshielded from conductive trace 20, tin metal 33 no longer interacts withthe copper metal of the trace.

The metal mitigating agent may be applied to the substrate of anelectronic assembly prior to the addition of electronic components tothe substrate. Alternatively, the metal mitigating agent may be appliedto the electronic assembly after one or more electronic components arecoupled to the substrate via electrical connections. The metalmitigating agent may be inert with respect to forming an alloy with puretin metal until and unless the electronic assembly is subjected toappropriate conditions for alloy formation (curing). Those conditionsmay be selected to form a tin metal alloy consuming substantially allpure tin metal present adjacent the metal mitigating agent, providedthat a sufficient amount of the metal mitigating agent is present toform such an amount of alloy. The tin alloy formed by interaction of tinmetal with the metal mitigating agent is resistant to whisker formation.This resistance may be due to the tin alloy having an overall tincontent of less than about 97% by weight. In one embodiment of themethod, the mitigating agent is a metal that is at least one of gallium,indium, gold, copper, aluminum, silver, magnesium, nickel, zinc, andlead.

With reference to FIG. 7, post-assembly curing of the electronicassembly results in an interaction between tin metal 33 and the metalfilm of mitigating agent 42, thereby forming a tin alloy 44 that isresistant to whisker formation. FIG. 8 depicts the electrical connectionof FIG. 7 after surface treatment with a reactive mitigating agent, andafter curing is substantially complete. As shown in FIG. 8, all pure tinmetal in electrical connection 22 has been converted to awhisker-resistant product. Specifically, all tin metal present has beenconverted to either tin alloy 44 or whisker-resistant product 36, asdescribed above.

In one version, the invention may include a whisker-resistant electronicassembly. The electronic assembly is resistant to the formation of tinwhiskers by virtue of being prepared by a process that includesproviding an electronic assembly having at least one electricalconnection that includes substantially pure tin metal, exposing the tinmetal to a mitigating agent selected to interact with the tin metal toproduce a product that is resistant to forming whiskers, and curing theelectronic assembly under conditions selected to promote the desiredinteraction. For example, the electronic assembly may be exposed to amitigating agent that is an oxidizing agent, the curing of theelectronic assembly may include heating the electronic assembly, and theproduct of the interaction of the tin metal with the oxidizing agent maybe a tin oxide.

Where the electronic assembly may have previously formed one or moretin-whisker-type structures before complete conversion/exhaustion of Snmetal and before treatment with an oxidizing agent, the preparation ofthe electronic assembly may include exposure of the electronic assemblyto an oxidizing agent, curing the electronic assembly, and then removingany tin-whisker-type structures that have been derivatized, or alteredchemically or metallurgically, such as by being converted to tin oxide,as set out at 44 in flowchart 10 of FIG. 1. Such structures wouldtypically be both fragile and non-conductive, and could be removed by anumber of methods, including for example vibration and/or exposure to ajet of high velocity gas, such as nitrogen gas or clean air.Alternatively or in addition, a gentle vacuum may be used to removefragments of oxidized tin whisker structures.

In yet another embodiment, the invention may include a whisker-resistantelectronic assembly that is resistant to the formation of tin whiskersby virtue of being prepared by a process that includes coupling at leastone conducting trace with tin metal with a mitigating metal that is atleast one of gallium, indium, gold, copper, aluminum, silver, magnesium,nickel, zinc, and lead. The mitigating metal is present in sufficientquantity that upon heating, it substantially converts the tin metal toan alloy that is resistant to whisker formation. In this embodiment, thetin metal may be converted to an alloy that is resistant to whiskerformation by heating the electronic assembly to a temperature betweenabout 25° C. and about 90° C.

The depiction of electrical connection 22 in FIGS. 4-8 should beconsidered to be illustrative only, and is intended to aid inunderstanding the invention. The relative size or thickness of a givencomponent or layer are illustrative and not necessarilyrepresentational.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

1. A method comprising: providing an electronic assembly, the electronicassembly having at least one electrical connection that includes atleast a surface that is substantially pure tin metal, the pure tin metalhaving a thickness; the method comprising: exposing the tin metal to atleast one mitigating agent selected to interact with the tin metal toproduce a product that is resistant to forming whiskers; wherein theelectronic assembly is exposed to the mitigating agent under appropriateconditions to promote the desired interaction.
 2. The method of claim 1,wherein the mitigating agent is a reagent, and the interaction with thetin metal is a chemical reaction.
 3. The method of claim 2, wherein themitigating agent is oxygen, and the interaction with tin metal isoxidation of the tin metal to at least one of SnO and SnO₂.
 4. Themethod of claim 3, wherein the appropriate conditions include heatingthe electronic assembly above room temperature.
 5. The method of claim4, wherein the appropriate conditions include heating the electronicassembly to between about 25° C. and about 90° C.
 6. The method of claim4, further comprising removing substantially all oxidized tin whiskersfrom the electronic assembly.
 7. The method of claim 2, wherein theappropriate conditions are sufficient to convert the tin metal surfaceof the at least one electrical connection to a product that is resistantto forming whiskers.
 8. The method of claim 2, wherein the appropriateconditions are sufficient to convert the surface and entire thickness ofthe at least one electrical connection to a product that is resistant toforming whiskers.
 9. The method of claim 1, wherein the mitigating agentis a metal, and the interaction with the tin metal is formation of a tinalloy that is resistant to forming whiskers.
 10. The method of claim 9,wherein the electronic assembly includes a mitigating agent that is aconducting layer of copper metal, and the tin alloy that is formedincludes one or more of Cu₆Sn₅ and CuSn₃.
 11. The method of claim 9,wherein the electronic assembly includes a conducting layer of coppermetal, and the mitigating agent is a metal film disposed between thecopper metal and the tin metal.
 12. The method of claim 9, wherein themitigating agent is at least one of gallium, indium, gold, copper,aluminum, silver, magnesium, nickel, zinc, and lead.
 13. The method ofclaim 9, wherein the formed tin alloy is less than about 97% tin. 14.The method of claim 1, wherein the electronic assembly further includesa circuit board and at least one electronic component coupled by the atleast one electrical connection.
 15. The method of claim 14, wherein themitigating agent is applied to the electronic assembly prior to couplingthe electronic component to the electrical connection.
 16. The method ofclaim 14, wherein the mitigating agent is applied to the electronicassembly after the electronic component is coupled to the electricalconnection.
 17. The method of claim 1, wherein the mitigating agent isapplied to the electronic assembly as a vapor or liquid
 18. The methodof claim 1, wherein a first mitigating agent is a reagent that interactswith the tin metal in a chemical reaction at the surface of the tinmetal; and a second mitigating agent is a metal film disposed betweenthe tin metal and the at least one electrical connection that interactswith an underside of the tin metal to form a tin alloy that is resistantto forming whiskers.
 19. An electronic assembly, prepared by a processcomprising: providing an electronic assembly having at least oneelectrical connection that includes substantially pure tin metal;exposing the tin metal to a mitigating agent selected to interact withthe tin metal to produce a product that is resistant to formingwhiskers; and curing the electronic assembly under conditions selectedto promote the desired interaction.
 20. The electronic assembly of claim19, wherein the mitigating agent is an oxidizing agent, curing theelectronic assembly includes heating; and the product is a tin oxide.21. The electronic assembly of claim 19, wherein the mitigating agent isa metal capable of forming an alloy with tin that is resistant toforming whiskers.
 22. The electronic assembly of claim 19, wherein theelectronic assembly includes a conducting layer of copper metal, and themitigating agent is a metal film disposed between the conducting layerand the at least one electrical connection.
 23. An electronic assembly,comprising: a circuit board substrate; a conducting trace comprisingcopper metal; and an electronic component coupled to the conductingtrace via tin metal; wherein the conducting trace and the tin metal arecoupled via a mitigating metal comprising at least one of gallium,indium, gold, copper, aluminum, silver, magnesium, nickel, zinc, andlead; the mitigating metal being present in sufficient quantity thatupon heating the mitigating metal substantially converts the tin metalto an alloy that is resistant to whisker formation.
 24. The electronicassembly of claim 23, wherein heating the electronic assembly to betweenabout 25° C. and about 90° C. converts the tin metal to an alloy that isresistant to whisker formation.